1. Field of the Invention
The present invention relates generally to an injection molding apparatus and, in particular, to a securing device for a removable nozzle tip.
2. Background
Conventional injection molding nozzles include a nozzle body and a separate tip. The tip is typically made of a highly thermally conductive material that transfers heat from heaters, coupled to the nozzle body, to a melt flowing through the tip. The tip may include a threaded portion to allow for direct coupling to threads on the nozzle body or, alternatively, a removable securing device may be provided to secure the tip in abutment with the nozzle body. The securing device typically includes a threaded portion that mates with threads on the nozzle body. In addition to securing the tip to the nozzle body, the securing device can be used to insulate the tip from a mold cavity plate.
The securing device is typically made of a low thermally conductive material, such as steel or titanium, to insulate the tip from the mold cavity plate. The securing device can further form a seal with the mold cavity plate to prevent pressurized melt from escaping into an insulative air space that surrounds the nozzle body.
An injection molding apparatus having the nozzle, described above, may be shut down for various reasons. For example, it may be shut down for regular scheduled maintenance and operation faults. In addition, it may be shut down to replace the tip with another type of tip having a configuration suitable for a different molding application.
Typically, to replace the tip access must be gained to a forward end of the nozzle. The securing device is released from the nozzle body, which also releases the tip. Following replacement of the tip, the securing device is reattached to the nozzle body, which secures the replacement tip in position.
Contraction and expansion of both the nozzle body and the securing device typically occurs each time the injection molding apparatus is shut down and restarted. This can cause the threads on the nozzle body and the securing device to rub against one another. This rubbing can result in thread galling. Unfortunately, the material that the threads are typically made from is particularly susceptible to this process.
Thread galling is prevalent in threads that are made from metals that generate a protective oxide surface film, such as stainless steel and titanium. As the threads of the nozzle body and the securing device rub against one another, the protective oxide film is broken down. Breaking down of the protective oxide film leads to increased adhesion between the threads. The adhesion causes removal and re-installation of the securing device from and to the nozzle body to become increasingly difficult. In an extreme case, thread galling may lead to seizing, or cold welding, of the threads. Once the threads have seized, any attempt to separate the securing device and the nozzle body will likely cause irreparable damage to the threads.
Conventional securing devices are not usually suitable for use with corrosive plastics, such as PVC (polyvinyl chloride) or TEFLON filled polyamide. Such plastics tend to corrode the securing device so that frequent replacement is necessary, which can be costly.
Conventional securing devices for nozzles can be used in some applications to provide a seal adjacent the nozzle, which can be used to limit the amount of molten material that resides around the mold gate. The seal is achieved through direct contact between the securing device and a surface of the mold, which is at a lower temperature.
Therefore, what is needed is a system and method that protect against thread galling of complementary threads located on a nozzle body and a securing device. Therefore, what is also needed is device that performs a sealing function made from a suitable material having a lower thermal conductivity than the material of the surrounding mold plate.